Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, some gaseous fuels have been recognized as attractive alternative fuels. For automotive applications, natural gas or petroleum gas may be compressed and stored as a liquid (liquefied petroleum gas, or LPG) in cylinders at saturation pressure. LPG predominantly consists of butane and propane, although the exact ratio may vary. LPG may be used with a direct injection and/or port injection fuel system. Under certain engine operating conditions, LPG injected from a fuel delivery system may be desired in either a liquid or a gaseous phase. However, as the temperature of the fuel in the fuel system changes, the LPG may be delivered to engine cylinders (e.g., combustion chambers) and/or fuel ports in an undesired phase. For example, the LPG may be in a liquid phase when a gaseous phase is required.
Other attempts to address maintain LPG in a desired phase include utilizing separate gaseous and liquid LPG storage tanks or adjust fuel delivery system parameters to maintain the LPG in one phase. One example approach is shown by Carter et al. in U.S. Pat. No. 8,443,785. Therein, a pump in the LPG fuel system is controlled to maintain the LPG in the liquid phase.
However, the inventors herein have recognized potential issues with such systems. As one example, under some engine operating conditions, LPG may be desired in the gaseous phase instead of the liquid phase. For example, during cold start conditions, LPG may be desired in the gaseous phase in order to reduce particulate matter, or soot formation. Thus, the system described above only provides a method for maintaining the LPG in a single liquid phase.
In one example, the issues described above may be addressed by a method for adjusting a fuel pressure in a fuel delivery system to deliver fuel in each of a liquid and a gaseous phase during different engine operating conditions. The fuel pressure may be based on a temperature, pressure in the cylinder in which fuel is injected into, and a desired phase of the fuel. In one example, the fuel is a gaseous fuel such as LPG. Adjusting the fuel pressure may include adjusting operation of a fuel pump in the fuel delivery system to deliver fuel to fuel injectors at the fuel pressure. For example, during a first condition such as a charge air temperature above a threshold, the fuel pump may be adjusted to increase the fuel pressure above a threshold pressure to transition from delivering the fuel in the gaseous phase to delivering the fuel in the liquid phase. During a second condition such as a cold start condition, the fuel pump may be adjusted to decrease the fuel pressure below the threshold pressure to transition from delivering the fuel in the liquid phase to the gaseous phase. In this way, LPG injected from the fuel delivery system may be injected in a liquid phase during a first set of engine operating conditions and injected in a gaseous phase during a second set of engine operating conditions. As a result, LPG delivered to engine cylinders may be maintained in a desired phase and transitioned between phases to increase engine efficiency and control and reduce engine emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.